Secondary batteries are used as power storage devices for portable equipment such as mobile phones and notebook personal computers, transport equipment such as automobiles and aircrafts, power leveling applications and the like. In either of these applications, it has been demanded to improve the energy density of the secondary batteries. At present, the energy density of lithium ion batteries is the highest among practical secondary batteries. Studies have been made to further improve the energy density of the lithium ion batteries while maintaining the safety of the lithium ion batteries. In part of such studies, all-solid-state batteries (each using a solid electrolyte in place of an electrolytic solution) are being studied as improvements of the lithium ion batteries.
The all-solid-state battery has a serial structure where negative electrode layers, solid electrolyte layers and positive electrode layers are repeatedly stacked together without using cupper wires etc. because all of battery component materials such as negative electrode material, solid electrolyte and positive electrode material are solid. The all-solid-state batteries are hence considered as being suitable for automotive applications and power storage applications. In particular, the all-solid-state oxide-based batteries in which negative electrode active material, solid electrolyte and positive electrode active material are each solid are expected to be effective in terms of improvement in safety and high-temperature durability in addition to improvement in energy density.
A garnet-type oxide Li7La3Zr2O12 (hereinafter also referred to as “LLZ”) has been proposed as an oxide material for forming a solid electrolyte layer (see Patent Document 1). Further, there has been proposed a technique for forming a composite metal oxide with a garnet-type structure as a lithium-ion conductive material by adding a sintering aid composed of a Al compound and a Si compound, or a Al compound and a Ge compound, to a mixed raw material of a Li compound, a La compound, a Zr compound and a Y compound etc., and then, sintering the mixed raw material at 1000 to 1200° C. (see Patent Document 2).
In order to obtain a dense sintered body of LLZ, however, it is necessary to perform sintering at a temperature of about 1100 to 1200° C. Under such sintering temperature conditions, there arise a quality problem of the occurrence of a change in the composition of the sintered body due to evaporation of lithium component during the sintering and an economic problem of the need for an expensive furnace capable of heating to high sintering temperatures. In the case of sintering the solid electrolyte together with the positive and negative electrode active materials, there also arises a problem of decomposition of the positive and negative electrode active materials under the above temperature conditions.
In view of these circumstances, it has been proposed to add a boron-containing compound to a calcined substance and fire the resulting material as a technique for obtaining a garnet-type oxide sintered body favorably even at a low sintering temperature.
For example, Patent Document 3 proposes a technique for producing a lithium-ion conductive oxide by preparing a liquid reactant containing a lithium compound, a lanthanum compound and a zirconium compound by so-called sol-gel process, drying and calcining the reactant and firing the resulting calcined pellet at a temperature lower than 1000° C. in the presence of a boron compound and an aluminum compound.
Patent Document 4 proposes a technique for producing a solid electrolyte by adding a flux to a lithium-ion conductive crystalline material and heating the resulting mixed material at 650 to 800° C. In this technique, the volume proportion of the flux is about 50 to 67%; and lithium borate or lithium borate in which boron is partially substituted by another element is used as the flux.
Furthermore, Patent Document 5 proposes a technique for producing a garnet-type ion conductive oxide material by adding lithium borate and aluminum oxide to a garnet-type ion conductive oxide and firing the resulting mixed material at 900° C. or lower, wherein the thus-produced garnet-type ion conductive oxide material includes: a base substance containing, as a main component, a composite oxide containing at least Li, La, Zr, Al, an element A (where A is one or more of Ca and Sr) and an element T (where T is one or more of Nb and Ta); and a grain boundary composition containing at least B and the element A.